A highly sophisticated computer worm that has spread through Iran, Indonesia and India was built to destroy operations at one target: possibly Iran's Bushehr nuclear reactor.

That's the emerging consensus of security experts who have examined the Stuxnet worm. In recent weeks, they've broken the cryptographic code behind the software and taken a look at how the worm operates in test environments. Researchers studying the worm all agree that Stuxnet was built by a very sophisticated and capable attacker -- possibly a nation state -- and it was designed to destroy something big.

Though it was first developed more than a year ago, Stuxnet was discovered in July 2010, when a Belarus-based security company discovered the worm on computers belonging to an Iranian client. Since then it has been the subject of ongoing study by security researchers who say they've never seen anything like it before. Now, after months of private speculation, some of the researchers who know Stuxnet best say that it may have been built to sabotage Iran's nukes.

Experts had first thought that Stuxnet was written to steal industrial secrets -- factory formulas that could be used to build counterfeit products. But Langner found something quite different. The worm actually looks for very specific Siemens settings -- a kind of fingerprint that tells it that it has been installed on a very specific Programmable Logic Controller (PLC) device -- and then it injects its own code into that system.

Because of the complexity of the attack, the target "must be of extremely high value to the attacker," Langner wrote in his analysis.

Langner is set to present his findings at a closed-door security conference in Maryland this week, which will also feature a technical discussion from Siemens engineers. Langner said he wasn't yet ready to speak to a reporter at length ("the fact of the matter is this stuff is so bizarre that I have to make up my mind how to explain this to the public," he said via e-mail) but others who have examined his data say that it shows that whoever wrote Stuxnet clearly had a specific target in mind. "It's looking for specific things in specific places in these PLC devices. And that would really mean that it's designed to look for a specific plant," said Dale Peterson, CEO of Digital Bond.

Langner thinks that it's possible that Bushehr may have been infected through the Russian contractor that is now building the facility, JSC AtomStroyExport. Recently AtomStroyExport had its Web site hacked, and some of its Web pages are still blocked by security vendors because they are known to host malware. This is not an auspicious sign for a company contracted with handling nuclear secrets.

Tofino Security Chief Technology Officer Eric Byres is an industrial systems security expert who has tracked Stuxnet since it was discovered. Initially he thought it was designed for espionage, but after reading Langner's analysis, he's changed his mind. "I guessed wrong, I really did," he said. "After looking at the code that Ralph hauled out of this thing, he's right on."

One of the things that Langner discovered is that when Stuxnet finally identifies its target, it makes changes to a piece of Siemens code called Organizational Block 35. This Siemens component monitors critical factory operations -- things that need a response within 100 milliseconds. By messing with Operational Block 35, Stuxnet could easily cause a refinery's centrifuge to malfunction, but it could be used to hit other targets too, Byres said. "The only thing I can say is that it is something designed to go bang," he said.

Whoever created Stuxnet developed four previously unknown zero-day attacks and a peer-to-peer communications system, compromised digital certificates belonging to Realtek Semiconductor and JMicron Technology, and displayed extensive knowledge of industrial systems. This is not something that your run-of-the-mill hacker can pull off. Many security researchers think that it would take the resources of a nation state to accomplish.

Bushehr is a plausible target, but there could easily be other facilities -- refineries, chemical plants or factories that could also make valuable targets, said Scott Borg, CEO of the U.S. Cyber Consequences Unit, a security advisory group. "It's not obvious that it has to be the nuclear program," he said. "Iran has other control systems that could be targeted."

Iranian government representatives did not return messages seeking comment for this story, but sources within the country say that Iran has been hit hard by the worm. When it was first discovered, 60 percent of the infected Stuxnet computers were located in Iran, according to Symantec.

Now that the Stuxnet attack is public, the industrial control systems industry has come of age in an uncomfortable way. And clearly it will have more things to worry about

"The problem is not Stuxnet. Stuxnet is history," said Langner in an e-mail message. "The problem is the next generation of malware that will follow."

Robert McMillan covers computer security and general technology breaking news for The IDG News Service. Follow Robert on Twitter at @bobmcmillan. Robert's e-mail address is robert_mcmillan@idg.com